Best Time to Dive Koh Tao: A Dive Instructor’s Guide

When people ask me about the best time to dive on Koh Tao, my answer is almost always the same: it depends on what you want to experience underwater.

I lived on Koh Tao for four years and worked as a dive instructor, diving the island on 800+ dives in every season. In my opinion, the best time to dive in Koh Tao is from March to May and September to October.

During these periods, conditions are usually at their most consistent. The sea is often glassy, visibility is excellent, and top dive sites like Chumphon Pinnacle are far more reliably accessible. That said, Koh Tao is very much a year-round dive destination, and I’ve had plenty of fantastic dives well outside these peak windows.

Below, I break down when you have the best chances of seeing whale sharks, when the island feels busiest, and why the so-called “off-season” can still be well worth considering.

If you’re just starting to plan your trip, you’ll also find more detail in my full Koh Tao diving guide, including how to choose the right dive center, what to budget, and which dive sites are truly worth prioritising.

Why March–May and September–October stand out for diving

These two periods stood out consistently during my time living and working on Koh Tao, but for different reasons.

March to May: Predictable and easy diving

From March to May, conditions are generally stable, and most dive sites can usually be dived on a daily basis. This makes it easy to plan even a short stay without worrying too much about cancellations or last-minute changes.

Visibility during this period is often good, but not always exceptional. Some days are fairly average, and plankton can reduce visibility at times, which is actually one of the main reasons why whale sharks show up around Koh Tao during these months.

This period also overlaps with the regional fishing ban. With no fishing for about a month, you can see massive schools of fish, especially along the outer reefs, sometimes surrounding you completely during the dive.

The biggest downside is the heat. Surface intervals can be exhausting, and staying hydrated is essential, particularly on full-day trips. Underwater, though, conditions are calm, relaxed, and easy diving.

What to expect from March to May:

• Calm seas: Minimal swell, relaxed boat rides, and easy entries. Ideal for longer trips out to Sail Rock.
• Good visibility: Especially at sites like Chumphon Pinnacle, 30 m+ is common on good days.
• Hot and dry weather: This is the hottest time of year. Conditions on land can feel intense, especially between dives.
• Whale shark season: Your best chances are around Chumphon, Southwest Pinnacle, and Sail Rock.
• Explosion of marine life: Thanks to the annual fishing ban during this period, you’ll see huge schools of fish, particularly along the outer reefs.

The downside?
The island is busy, both underwater and topside, especially around Songkran (Thai New Year). If you’re looking for quiet dive sites and empty boats, this isn’t the time to come.

September to October: Best overall conditions

September to October often delivers the best diving conditions of the entire year on Koh Tao. In my experience, this is when everything comes together. The wind drops, the sea settles, and the water clears up to the point where visibility regularly reaches 40 metres or more.

At this time of year, you can sometimes see the sandy bottom of sites like Chumphon Pinnacle or Southwest Pinnacle straight from the surface. On top of that, dive sites feel noticeably quieter, days on the boat are more relaxed, and the whole experience feels far less rushed.

It often feels like the calm before the storm, just before the monsoon sets in around November. That’s why this is my personal favourite time to dive on Koh Tao, especially for relaxed, unhurried fun dives.

What to expect in September and October:

• Flat seas: often the calmest conditions of the entire year
• Crystal-clear water: especially at deeper sites, visibility can be outstanding
• Whale sharks are back: never guaranteed, but chances are good during this period
• Fewer people: shoulder season means fewer divers on the boats and underwater
• Comfortable temperatures: a bit cooler than the peak dry season, but still warm and sunny

The only downside: The rainy season is just around the corner. Towards the end of October, you might get the occasional shower, but it’s usually short-lived and diving tends to carry on as normal.

September and October are also great months to explore Koh Tao’s best snorkelling spots on non-diving days.

Koh Tao Diving conditions by season

Diving conditions on Koh Tao change noticeably throughout the year. Below you’ll find a realistic breakdown of what to expect in each season and which months are actually worth it, depending on what kind of diving you’re looking for.

Rainy season (November – January)

The rainy season is the most unpredictable time to dive on Koh Tao. During these months, the northeast monsoon brings stronger winds and, at times, significant swell. Sea conditions can change completely from one day to the next.

Diving is generally possible, but never guaranteed.

On good days, dives run almost normally. On stormy days, boats stay in the harbour. Whether diving happens at all is often decided only in the morning, after checking wind and wave conditions.

The reality is: top sites like Chumphon Pinnacle or Sail Rock are rarely visited during this period, as they are completely exposed. Instead, diving usually shifts to more sheltered sites in the southwest of the island, such as Junkyard Reef or White Rock.

What you can realistically expect:

• Visibility is often reduced or changes a lot from day to day
• Fewer chances for big stuff
• Getting on and off the boat can be pretty sporty when there’s swell
• Limited site choice. It’s not unusual to dive the same sheltered site three days in a row

Rainy season diving at a glance

Remember, Koh Tao’s monsoon doesn’t run for three solid months. The rough weather usually comes in shorter phases, often two or three weeks at a time. Before and after that, you can get really decent days. You just can’t plan for them.

My honest take on the rainy season: If you’re on the island for four weeks and can stay flexible, it can work. If you’ve only got a few days, or you’re trying to squeeze in a specific course like the Deep Diver, this season is often frustrating. During these months, having a plan B on land isn’t optional, it’s essential. I’ve put together my best tips for things to do on Koh Tao so your trip still works out even when the weather doesn’t.

Dry season (February – May)

The dry season is one of the most reliable times to dive on Koh Tao. Most days you get calm seas, little wind, and solid visibility. It’s an easy time to dive.

From February to May, conditions are generally warm and stable. This is also when whale sharks tend to show up more often, even though there are never any guarantees. Because the weather plays along, you can usually reach most dive sites without much trouble.

What you can realistically expect:

• Good visibility: often around 30 metres, especially in March and April
• Whale shark season: best chances at deeper sites like Chumphon Pinnacle and Southwest Pinnacle
• Busier boats: it’s high season, so popular sites are rarely quiet
• Heat: it gets hot on land, and long surface intervals can feel draining without shade

Dry season diving at a glance

Windy season (June – October)

During these months, the wind picks up, usually from the southwest, and the sea can get a bit choppier. Diving is still very much possible and often very good.

The closer you get to the end of the season, the better it gets. September and October often bring the clearest water of the entire year on Koh Tao. This is when everything starts to line up again.

This is where wind really starts to matter: from June to August, we often switch to the sheltered east coast of the island (for example, Hin Wong Bay or Lighthouse Bay). While Sairee can be rough, these spots are often completely flat. Longer trips out to exposed sites like Sail Rock can be pretty uncomfortable at this time.

What you can realistically expect:

• Crystal-clear water: visibility up to 40 metres in September and October
• More breeze on land: the wind takes the edge off the heat
• Quieter dive sites: after the August high season, September feels noticeably calmer underwater
• Changing conditions: in mid-summer (July/August), visibility can swing quickly with stronger winds

Windy season diving at a glance

How to plan your dive trip by season

Here are a few tips to help you choose the best time for your dive trip to Koh Tao, depending on what you’re looking for.

Best time for near-perfect diving conditions

If calm seas and top visibility are your priority, two periods stand out:

• March to May: Visibility often pushes past the 30-metre mark, and whale shark chances are genuinely good. The sea is usually flat, which makes diving easy and relaxed. The downside: it gets very hot on land, and April in particular can be extremely busy due to Songkran and Easter holidays.
• September to October: My personal favourite time to dive on Koh Tao. The wind drops, the sea goes completely calm, and visibility is better than at any other time of year. Whale sharks start showing up again, and because high season is over, many dive sites feel surprisingly quiet.

Best time to avoid the crowds

If you’d rather dive quieter sites and don’t enjoy packed boats, these months work best:

• May to June & September to October: Fewer tourists on the island, but still very good diving conditions. September and October stand out again, with excellent visibility and noticeably emptier boats.
• November: This is when low season officially starts. You might get some rain, but dive sites are often almost empty. If you’re flexible and don’t mind a bit of uncertainty, it can be a great time for relaxed, unrushed dives.

Best time if you’re on a smaller budget

If you’re looking for cheaper accommodation and good dive deals, these months usually work best:

• May–June & September–November: The island is quieter, hotels tend to drop their prices, and diving conditions are still very solid.

When you should book in advance

During high season, it’s worth booking accommodation and dive courses early, especially if you’re travelling on a budget:

• April & July–August: These are official high-season months. Many dive centres and hotels fill up quickly.
• Christmas & New Year (through early January): One of the busiest times of the year on Koh Tao. If you want to be here over the holidays, booking ahead is strongly recommended.

How the season affects dive site choice

Not all dive sites are accessible year-round. Conditions decide where we dive.

• Monsoon season (November–January): When the sea gets rougher, deeper and more exposed sites like Sail Rock and Chumphon Pinnacle are often hard to reach. During this time, diving usually shifts to more sheltered sites along Koh Tao’s southwest coast.
• Windy season (June–August): With strong westerly winds, diving often moves to the east side of the island, where conditions stay calmer. My favourite east-coast sites are Hin Wong Pinnacle and Lighthouse Bay, and this is often when they’re at their best.